ArrayList creation with Java 8 Lambda

Question

I would like to find a one-liner expression in Java that allows to instantiate an ArrayList and to insert some specific elements inside it.

For example, an expression that creates an ArrayList containing the numbers in the interval [0, n] divisible by 3.

I manage to insert the filtered elements into the ArrayList, but only using the forEach expression in the following way:

ArrayList<Integer> numberList = new ArrayList<Integer>;
IntStream.range(0, n + 1)
         .filter(number -> number % 3 == 0)
         .forEach(number -> numberList.add(number));

As you can see, it's necessary to separately instantiate the ArrayList and then insert the elements.

In Python, we can use the following one-liner:

numberList = [number for number in range(0, n + 1) if number % 3 == 0]

Is there something equivalent or similar to the code below in Java 8?

Thanks in advance.

What about collecting to a List?

Kayaman
– Kayaman

2018-01-30 13:44:50 +00:00
Commented Jan 30, 2018 at 13:44 — Kayaman
– Kayaman, Commented Jan 30, 2018 at 13:44

Junior Dussouillez · Accepted Answer · 2018-01-31 06:58:38Z

6

You can use a collector to add the elements in a list :

int n = 10;
List<Integer> list = IntStream.range(0, n + 1)
      .filter(number -> number % 3 == 0)
      .boxed() // converts IntStream to Stream<Integer> (primitive type to wrapper type)
      .collect(Collectors.toList());

Edit: As Aominè said in the comments, you can also use IntStream.rangeClosed(0, n) instead of IntStream.range(0, n + 1)

edited Jan 31, 2018 at 6:58

answered Jan 30, 2018 at 13:45

Junior Dussouillez

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4 Comments

Alessandro Bardini Over a year ago

Thank you, this is perfect. I tried to use the collect() method without using boxed() before. With boxed() everything works fine

Alexis C. Over a year ago

@AlessandroBardini In this case you would have to do .collect(ArrayList::new, List::add, List::addAll); which would be the equivalent of .boxed().collect(toCollection(ArrayList::new));

Mark Jeronimus Over a year ago

Both should have the same performance. The former (with boxed) is neater.

Ousmane D. Over a year ago

more suitable to use IntStream.rangeClosed(0, n) rather than IntStream.range(0, n + 1) in this case.

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